That depends on the brand. The lower priced brands, yes, those can be SATA, the more vertically integrated companies also make custom PCBs that just have USB-C without any SATA interface exposed internally.
It's probably feasible to make a "mass storage USB in, SATA protocol out" smart adapter board.
I see, but if you plan on shucking you obviously get ones you know are able to.
I've shucked WD MyBook drives, just a plain SATA inside. I guess that it's cheaper to have a stock drive and a cheap SATA-USB adaptor in a shell than do custom electronics. I've not heard of any that are otherwise, but I've only done a few. I suppose it's possible that they could solder them in or have custom electronics but I would have thought that rare. It's frequently discussed on Reddit too, so there's plenty of folk doing this.